Spatial and Temporal Variability of SeaWiFS Chlorophyll-a Distributions West of the
Antarctic Peninsula: Implications for krill production
Marina Marrari*, Kendra L. Daly and Chuanmin Hu
College of Marine Science
University of South Florida
140 Seventh Avenue South
St. Petersburg, FL 33701, USA
* Corresponding author: mmarrari@marine.usf.edu; Tel: 727-553-1207; Fax: 727-553-1186
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Abstract
SeaWiFS chlorophyll-a distributions and sea ice coverage were investigated in the western
Antarctic Peninsula region (45 - 75 ºS, 50 - 80 ºW) [this is the entire image, but perhaps much
larger than your region – narrow it down?]in relation to the reproductive patterns and
recruitment success of Antarctic krill Euphausia superba The Bellingshausen Sea and
Marguerite Bay in the south of the study area are consistently more productive during austral
spring and summer than the northern sectors, including the continental shelf along the
Antarctic Peninsula and the western Scotia Sea. These predictable [?]elevated phytoplankton
abundances likely provide krill with the food levels required to ensure a successful
reproduction and larval survival. The interannual variability observed in krill recruitment
between fall 2001 and 2002 can be related to differences in the timing and extent of
phytoplankton blooms during the preceding spring - summer season. The 2000/2001 season
was generally more productive than 2001/2002, in coincidence with elevated krill numbers in
oceanic and coastal areas during fall. In contrast, 2001/2002 showed reduced chlorophyll and
lower overall krill abundances during fall throughout the study area. Interannual differences
in sea ice conditions in Marguerite Bay most likely also contributed to the variable krill
numbers observed in the area. Sea ice persisted throughout spring and summer 2001/2002,
preventing the accumulation of chlorophyll in the water column, which is the main food
source for krill in coastal waters during the productive months. In contrast, sea ice melted
earlier in 2000/2001, allowing elevated phytoplankton blooms to develop within Marguerite
Bay. Blooms in off-shelf waters during spring were not related to the retreat of the ice edge
in our study area, except possibly in the southernmost regions. It is likely that other shelfbreak processes, such as the upwelling of iron-rich deep waters, control the development of
these ice edge phytoplankton blooms. Both the occurrence of the above-average chlorophyll
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concentrations during spring and summer and favorable sea ice conditions in coastal areas
most likely supported the increased krill abundances observed during fall 2001 in waters west
of the Antarctic Peninsula.
Keywords: Antarctic krill, chlorophyll, climatology, Euphausia superba, SeaWiFS, Southern
Ocean, Antarctic Peninsula, Bellingshausen Sea.
Introduction
The waters west of the Antarctic Peninsula support relatively high densities of
phytoplankton, zooplankton, and upper trophic level predators, and the region is considered to
be one of the most productive of the Southern Ocean (Deibel and Daly, in press). The
Antarctic krill, Euphausia superba, plays a key role in this ecosystem as one of the primary
pelagic herbivores and prey for many predators, including whales, seals, penguins, seabirds
and fish. In addition, krill have been commercially harvested in this region since the 1960s
and are still the subject of an active fishery by several nations (Ichii, 2000). The large krill
population appears to be maintained by occasional strong year classes, with often poor
recruitment in the intervening years (Siegel and Loeb, 1995; Quetin and Ross, 2003). The
suite of physical and biological factors that govern krill reproduction and recruitment,
however, remain poorly known.
Successful krill reproduction (November - March) and larval survival during summer
require an adequate food supply (Ross and Quetin, 1983; 1989). Adult females may require
above average phytoplankton concentrations (1 - 5 mg chl m-3) to initiate reproduction (Ross
and Quetin, 1986) and relatively high chlorophyll concentrations (> 0.5 mg chl m-3) to sustain
multiple spawning throughout the summer (Nicol et al., 1995). Early primary production in
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polynyas, near ice edges or shelf breaks, and summer blooms onshelf may provide important
food sources for young larvae. Laboratory experiments suggest that first-feeding larvae may
not survive delayed food or when only small flagellates are available as a food source (Ikeda,
1984; Ross and Quetin, 1989). Furthermore, enhanced food availability allows larvae to
achieve faster growth and developmental rates, thereby obtaining a larger size and being in
better condition to survive overwinter (Daly, 2004). Thus, knowledge about differences in
the timing, extent, and evolution of phytoplankton blooms is important for understanding
interannual variability in krill recruitment.
The Southern Ocean Global Ocean Ecosystems Dynamics Program (SO GLOBEC)
investigated the physical and biological factors that influence the growth, recruitment, and
overwintering survival of Antarctic krill in the vicinity of Marguerite Bay, west of the
Antarctic Peninsula, during austral fall and winter of 2001 and 2002. Large differences in
abundances of larval and juvenile krill were observed between these two years (Fig. 1) (Daly,
2004). During fall 2001, larvae were very abundant, with younger stages dominant offshelf
and older stages dominant onshelf. Abundances of up to 343 ind. m-3 were observed offshelf,
the highest ever recorded for the area. There was a wide range of larval stages offshelf,
whereas only older individuals were observed near the coast, indicating that most larvae were
hatched in oceanic waters and later transported into coastal areas. Onshelf larvae were in
better condition than offshelf larvae, suggesting that there was enhanced food availability near
Marguerite Bay during the preceding summer. Few juveniles were observed anywhere.
During fall 2002, relatively high concentrations of young larvae were again detected in
oceanic waters (< 1 - 41 ind m-3), although overall abundances were an order of magnitude
lower than in 2001 and all stages were scarce in coastal areas. Offshelf larvae were primarily
calyptopis while onshelf larvae were mostly calyptopis and FI and II stages. Juveniles also
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were relatively abundant on the inner and middle shelf in the vicinity of Marguerite Bay (1 83 ind. m-3), indicating a successful recruitment from the 2001 larval population. These
results prompted us to investigate the environmental conditions that contributed to the large
krill reproduction and subsequent high larval densities particularly during austral spring and
summer 2000/2001.
Herein, we investigate interannual chlorophyll patterns and dynamics west of the
Antarctic Peninsula using SeaWiFS ocean color data between 1997 and 2004, with special
emphasis on the Marguerite Bay region to better understand the conditions that make it a
suitable habitat for krill. We generate a climatology of the surface chlorophyll field and
examine the anomalies corresponding to summer - fall 2000/2001 and 2001/2002 with respect
to the interannual variability in krill abundances during SO GLOBEC. We also investigate
the effects of sea ice extent on the timing and location of phytoplankton blooms west of the
Antarctic Peninsula.
Materials and Methods
The study area consisted of the coastal waters west of the Antarctic Peninsula and
adjacent deep waters in the Drake Passage and Bellingshausen Sea (45 - 75 S and 50 - 80
W), as chlorophyll in these areas are most likely to influence regional krill populations (Fig.
2).
The chlorophyll dataset includes 6606 SeaWiFS daily Level 2 files (~ 1 km/pixel near
nadir) between September 1997 and December 2004 obtained from NASA Goddard Space
Flight Center (http://oceancolor.gsfc.nasa.gov). These data were collected by all ground
stations, as well as occasional satellite onboard recording over the area and processed using
the most recent algorithms and software package (SeaDAS4.8). The level 2 data were
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mapped to a rectangular projection with approximately 1 km2/pixel for the western Antarctic
Peninsula region (Fig. 2). The parameter used in this study is the surface chlorophyll-a
concentration (mg m-3) derived from the OC4v4 empirical band-ratio (blue versus green)
algorithm (O’Reilly, 2000). Comparison with in situ chlorophyll-a values measured by HPLC
showed that for > 90% of the waters in the Southern Ocean (chlorophyll-a between 0.05 and
1.5 mg m-3) SeaWiFS chlorophyll-a is accurate (Marrari et al., in press). For higher
concentrations, the accuracy of the data was not verified but it is expected to be consistent.
A 7-year bi-weekly climatology of chlorophyll-a distributions was generated from the
mapped Level 2 data from September 1997 to December 2004 (Fig. 3). Bi-weekly composite
images were also generated, and the anomaly images were generated as the difference
between the bi-weekly composites and their corresponding climatological bi-weekly
composites.
Fourteen regions were defined within the study area, each representing different
geographic location and oceanographic conditions (Fig 4a). The average (median)
chlorophyll concentration for each region was estimated during the 2000/2001 and 2001/2002
productive seasons (September - March) and plotted over time in relation to the climatology
(Fig. 4b). In addition, the anomalies (mg chl m-3 above or below the climatology) within the
entire study area were mapped during the spring - summer of 2000/2001 and 2001/2002
(November - February) (Fig. 5).
The mean location of the ice edge within the study area during October, November
and December of 2000 and 2001 was determined using a two-dimensional linear interpolation
of monthly ice concentration on a 25 km resolution grid. Monthly averaged gridded ice
concentrations generated using the NASA Team algorithm and Nimbus-7 SMMR and DMSP
SSM/I passive microwave data were obtained from the National Snow and Ice Data Center
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(Cavalieri et al., 2005). The ice edge was considered to be the location where sea ice
concentration was ≤ 15% (Gloersen et al., 1992). The mean location of the ice edge during
these months was superimposed over the concurrent biweekly SeaWiFS chlorophyll images.
In addition, the location of the ice edge during the preceding month was also plotted in order
to evaluate changes in chlorophyll concentrations within the region of the ice edge retreat
(Fig. 6).
The daily area free of sea ice (km2) in the northern and southern sections of
Marguerite Bay was estimated from satellite data following the methods described in Arrigo
and van Dijken (2003) (Fig 7a). A daily climatology from 1997 through 2004 was calculated
as the mean daily ice-free area within each sub-region. A running average was applied to
reduce the daily variability. The climatology was plotted in relation to the daily ice-free areas
for 2001 and 2002 in Figure 7.
Results
Biweekly climatological patterns of chlorophyll-a in waters west of the Antarctic
Peninsula between September and March 1997 - 2004 (Fig. 3) indicate that offshore waters in
the Antarctic Circumpolar Current (ACC) typically had relatively low concentrations (0.1 0.2 mg m-3). In contrast, the highest concentrations consistently occurred in coastal waters in
the vicinity of Marguerite Bay and to the south in waters of the eastern Bellingshausen Sea.
These phytoplankton blooms persisted throughout the summer (December - March), with
mean values in Marguerite Bay during early January of 2.78 ± 10.09 mg m-3. [standard
deviation is much higher than mean?] A range of intermediate values (0.39 ± 1.59 mg m-3)
was observed during the same period over the more northern continental shelf regions west of
the Antarctic Peninsula and downstream in the Scotia Sea. The spatial and temporal changes
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in chlorophyll-a patterns suggest that the biomass accumulations initially occur during
October and November in offshelf waters, mainly in the Bellingshausen Sea and to a lesser
extent near the shelf break in the vicinity of the Shetland Islands at the northern end of the
Antarctic Peninsula (Fig. 3). As the season progresses (mid-December), phytoplankton
blooms develop onshore especially in the vicinity of Marguerite Bay, where they remain well
established until early April. Thus, oceanic and coastal areas in the Bellingshausen Sea and
coastal Marguerite Bay waters include particularly high chlorophyll concentrations during
spring and summer in comparison with any other area west of the Antarctic Peninsula.
Of the fourteen selected regions (Table 1, Fig. 4a), regions 1 - 6 represent offshelf
oceanic regimes with depths greater than 2000 m, while regions 7 and 8 are located over the
continental shelf slope, defined as the area between 500 and 2000 m water depth. Regions 9,
10 and 11 represent coastal waters along the Antarctic Peninsula shelf and regions 13 and 14
are located in Marguerite Bay. Region 12 includes both coastal and oceanic waters in the
Scotia-Weddell confluence area.
Both the median chlorophyll distributions in these areas (Fig. 4) and the anomaly data
(Fig. 5) indicate that, overall, 2000/2001 had higher chlorophyll concentrations than
2001/2002, particularly in the Bellingshausen Sea, Marguerite Bay, and other coastal areas
along the Peninsula. The median chlorophyll concentrations for the 14 regions reveal
relatively small variations from the climatology in offshelf regions of the ACC, Drake
Passage, and the Scotia Sea (regions 1 - 6), although chlorophyll concentrations during
2000/2001 in offshore waters of the Bellingshausen Sea (regions 1 and 4) showed a moderate
increase with respect to typical values (Fig. 4b). Shelf break and coastal regions showed
increased production relative to the climatology with median values in some regions
exceeding the average by up to 5 mg m-3. In the Bellingshausen Sea (regions 7 and 9),
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chlorophyll concentrations were generally elevated during both seasons, with high variability
observed both between and within years. The summer of 2000/2001 showed the highest
median concentrations with values reaching 1.7 mg m-3 in February, whereas chlorophyll
concentrations during 2001/2002 were generally lower. Shelf break and coastal regions along
the Antarctic Peninsula (regions 8, 10 and 11) also had elevated chlorophyll concentrations in
relation to offshore areas, although the variations with respect to the climatology were less
evident than in the Bellingshausen Sea and median values never exceeded 0.63 mg chl m-3.
Marguerite Bay (regions 13 and 14) had the highest median chlorophyll concentrations in
comparison with any other region analyzed. In September, average values in northern
Marguerite Bay were 0.3 - 0.63 mg m-3, but during September 2000, chlorophyll reached 5.62
mg m-3. For the same region, 2002 values were consistently near or below the 7-year mean.
Sea ice prevented satellite data collection during most of the 2001/2002 summer in Southern
Marguerite Bay, but 2000/2001 showed high median values of up to 3.21 mg chl m-3 from
late December through February, a 270% increase with respect to the average conditions (i.e.
1.19 mg m-3).
Similar patterns can be observed from a temporal perspective considering the anomaly
data (Fig. 5). During late November, both years showed strong positive anomalies west of the
Antarctic Peninsula. In 2000/2001, however, these above average chlorophyll concentrations
were observed over the continental shelf, while in 2001/2002 positive anomalies were
observed in oceanic waters and in the Bellingshausen Sea. By December, 2000/2001 showed
positive anomalies in Marguerite Bay and in coastal areas along the entire Peninsula, while
offshore areas had negative values. In contrast, 2001/2002 had below average chlorophyll
concentrations in the Peninsula coastal waters and above average concentrations in oceanic
areas. In January and February 2001, widespread and strong positive anomalies were still
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present in the Bellingshausen Sea, Marguerite Bay, and along the Peninsula shelf, whereas
ACC waters showed average conditions. In January and February 2002, overall conditions
had progressed toward a mean state, and strong positive values only were observed in a
narrow band along the ice edge (black area) in the Bellingshausen Sea during January. It is
important to note, however, that due to extensive cloud cover, the number of valid data points
available for late February 2002 is considerably lower than that for 2001. In summary, the
2001/2002 season started with higher than normal chlorophyll concentrations offshelf, but by
January, conditions had progressed toward an average state. Even though the 2000/2001
spring - summer began with weaker positive anomalies than 2001/2002, by January many
areas showed increased production. Throughout the rest of the summer, 2000/2001 showed
very widespread positive anomalies, particularly in the Bellingshausen Sea, Marguerite Bay,
and along the continental shelf.
The location, timing, and extent of sea ice were examined in relation to chlorophyll
concentrations to better understand the relationship between sea ice and phytoplankton
blooms (Fig. 6). Chlorophyll concentrations were highly variable in relation to the receding
ice edge in our study area during 2000/2001 and 2001/2002. In offshelf waters during spring,
phytoplankton blooms occurred both adjacent to the ice edge and/or in waters previously
covered by sea ice 2 - 4 weeks earlier. For example, during September 2000 and 2001, the ice
edge was located in oceanic waters of the ACC. By October, the ice margin had retreated
considerably and occurred closer to the coast. Chlorophyll had not increased significantly at
the September ice edge locations by October (Fig. 6a and 6b, top 2 panels), suggesting that on
average, October was too early in the productive season for any significant chlorophyll
accumulations to occur within the ice edge zone. During November, however, chlorophyll
concentrations had increased significantly in this region, reaching ~ 5 mg m-3, but presumably
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were too far from the ice edge to be influenced by ice processes (Fig. 6a and 6b, center
panels). During November, the ice edge had receded onshelf in the mid-Antarctic Peninsula,
but remained offshore of the southern Peninsula in both 2000 and 2001. Subsequently during
December, enhanced chlorophyll concentrations occurred within the region of ice retreat. In
some regions the 2 phenomena seem to coincide in space after a periodic time lag of 2 to 4
weeks. In addition, areas that were never influenced by sea ice also showed increased
production. For example in November and December of 2001 (Fig. 6b), the ice edge
occupied coastal areas of the Bellingshausen Sea and along the Antarctic Peninsula to Anvers
Island (Fig. 2). Even though ice never occupied the northern end of the Peninsula, elevated
chlorophyll concentrations were observed along the shelf-break and in coastal areas. Thus,
processes other than the retreat of the ice edge likely influenced phytoplankton dynamics in
this area.
Sea ice coverage in Marguerite Bay also showed strong differences between the spring
- summer of 2000/2001 and 2001/2002. During summer and early fall (January - April),
typical values of ice-free areas range from approximately 9,000 to 11,000 km2 in northern
Marguerite Bay and from ~ 4,500 to 7,500 km2 in southern Marguerite Bay (Fig. 7a). A
comparison of the climatology and 2001 and 2002 daily ice-free areas (km2) indicates that
2002 had above average sea ice in both the northern and southern sectors throughout the
spring, summer, and fall (Fig 7b and 7c). In addition, ice formed earlier in 2002 than in 2001.
In contrast, 2001 had sea ice values significantly below the 8-year mean, particularly from
January through July, as indicated by the unusually large ice-free areas observed both in the
northern and southern sectors. In 2001 these values reached approximately 12,000 and
11,500 km2 in the northern and southern regions respectively. On the other hand, the areas
free of ice only reached 6,000 - 9,000 km2 in the northern and 0 - 2,000 km2 in southern
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sectors during the same months in 2002, suggesting an especially extensive sea ice cover.
During winter (starting in mid-July or Julian Day ~ 200), sea ice conditions were similar for
both years, although, as mentioned above, sea ice occupied both the northern and southern
sectors considerably earlier in 2002.
Discussion
Chlorophyll concentrations in waters west of the Antarctic Peninsula showed great
temporal and spatial variability during spring and summer between 1997 and 2004. There
have been numerous reports of high chlorophyll values over the continental shelf west of the
Peninsula, with some concentrations occasionally in excess of 30 mg m-3 (e.g., Holm-Hansen
et al., 1989; Smith et al., 1998; 2001). These findings support the belief that the Antarctic
Peninsula is one of the more productive regions in the Antarctic. However, very few studies
to date involve the Bellingshausen Sea (Smith et al., 1992; Savidge et al., 1995) and data from
the Marguerite Bay have been scarce. Our results indicate that chlorophyll concentrations in
Marguerite Bay and the Bellingshausen Sea are consistently higher than in any other area
analyzed, making these southern Antarctic Peninsula regions vitally important in terms of
phytoplankton production. In addition, chlorophyll accumulations in these areas occur earlier
in the spring and persist longer throughout the summer, further contributing to the higher
overall seasonal production in the southern sector compared to the more northern regions
along the Antarctic Peninsula. Thus, Marguerite Bay and the Bellingshausen Sea need to be
considered in future primary productivity and ecosystem assessments.
Krill reproduction and recruitment may be strongly influenced by the early availability
and seasonal persistence of elevated chlorophyll concentrations in the Bellingshausen Sea and
Marguerite Bay. Several processes have to take place in order for a successful recruitment of
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krill to occur along the Antarctic Peninsula: (1) successful reproduction and larval survival
during summer, (2) advection of offshelf larvae onto the continental shelf, (3) retention of
larvae on the shelf, and (4) larval survival overwinter (Daly 2004). West of the Antarctic
Peninsula, E. superba typically reproduces during late spring and summer, from November
through March (Siegel 1988) and requires above-average food concentrations (1 - 5 mg chl m3
) (Ross and Quetin, 1986; Nicol et al., 1995). During the spawning season, adult females
migrate near the vicinity of the shelf break in spring where they spawn in oceanic waters
(Siegel, 1988). Eggs hatch at depth and the newly hatched larvae (naupliar stages) swim to
the surface before metamorphizing into the first feeding stage, calyptopis I (CI). It is then
crucial for these larvae to encounter an adequate food supply in the euphotic zone within
about 10 days otherwise they will not survive (Ross and Quetin, 1986). The chlorophyll
concentrations required for reproduction and a dependable availability of food for larvae are
conditions most commonly found in Marguerite Bay and the Bellingshausen Sea. On-shelf
advection processes (Dinniman and Klinck, 2004; Klink et al., 2004) and retention
mechanisms (Hofmann et al., 1996; Beardsley et al., 2004) have been documented in the
vicinity of Marguerite Bay. Offshelf larvae that are advected onto the shelf will find an
enhanced food supply and more favorable environmental conditions. Larvae that are not
advected onto the shelf are transported eastward away from the Antarctic Peninsula and into
the Scotia Sea by the Antarctic Circumpolar Current (Hofmann ref (?)).
Field evidence indicates that during 2000/2001 krill reproduction started relatively
early and continued for an extended period. A wide range of larval stages were observed in
offshelf waters during fall 2001, including older larvae (furcilia VI), a dominant furcilia I (FI)
mode, and significant numbers of calyptopis III (C3) (Daly 2004). Based on experimentally
determined growth rates (Ikeda,1984), FI’s are estimated to be about 63 days old and,
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therefore, likely originated from a late February - early March reproductive event. Surface
velocities in the ACC can reach 0.25 - 0.4 m sec-1 but decrease monotonically with depth
(Klinck and Nowlin, 2001). In addition, mesoscale meanders and eddies may act to reduce
this transport rate. Assuming a mean eastward current velocity of the ACC of ~ 0.1 m sec-1,
the spawning location possibly occurred in offshelf waters of the Bellingshausen Sea,
presumably in the area bounded by 85 - 80 W, and 65 - 70 S. Thus, offshelf waters of the
Bellingshausen Sea may have supported an early and extended krill reproduction in
2000/2001.
Variability in the recruitment of larval krill populations west of the Antarctic
Peninsula may be related to differences in food supply during the preceding spring - summer.
During SO GLOBEC years, 2000/2001 was more productive than 2001/2002, with strong
widespread positive chlorophyll anomalies during summer in the Bellingshausen Sea,
Marguerite Bay, and other coastal areas. These elevated chlorophyll levels likely provided
krill with the food required for a successful reproduction and recruitment, which resulted in
the elevated larval abundances observed during fall 2001 and the high juvenile abundances
recorded in coastal waters during fall 2002. In contrast, even though 2001/2002 showed
above average chlorophyll concentrations early in the season, conditions shifted toward a
mean state by February - March. The lack of older-stage larvae and relatively lower larval
densities in 2001/2002, suggests that there was either delayed reproduction of adult females in
summer and/or a lower larval survival. Thus, the presence of above average chlorophyll
concentrations in offshore waters of the Bellingshausen Sea and Marguerite Bay during
summer seems critical for the successful reproduction of the Antarctic krill population that
will later inhabit coastal waters along the western Antarctic Peninsula.
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Sea ice extent and duration influence the timing and location of phytoplankton
blooms. Sea ice reduces light penetration into the water column and prevents phytoplankton
blooms from developing during summer, which in turn leads to lower food concentrations
available for herbivores. Although sea ice extent and length of the sea ice season in the
Bellingshausen Sea and along the Antarctic Peninsula have shown a clear tendency to
decrease over the past 25 years (Parkinson, 2002; Ducklow et al., in press), high interannual
variability in sea ice is still observed. The summer - fall of 2001 showed an unusually low
sea ice extent in Marguerite Bay, while 2002 was characterized by a particularly extensive
and persistent sea ice cover, a pattern that was observed in other areas west of the Antarctic
Peninsula. Ducklow et al. (in press) analyzed 14 years of sea ice extent data near Palmer
Station in the vicinity of Anvers Island (1991 - 2004), and found that 2001 had the lowest
(69,932 km2) winter sea ice extent of all years analyzed, while 2002 had the highest (109,936
km2) (mean = 91,112 km2). The early retreat of sea ice in Marguerite Bay in spring of 2001
and subsequent presence of large phytoplankton blooms in ice-free waters during summer
probably contributed to the elevated krill concentrations observed in the area during the
following fall. In contrast, the persistent presence of sea ice in Marguerite Bay during
summer - fall 2002 resulted in overall lower chlorophyll concentrations in coastal surface
waters, which probably contributed to lower krill abundances.
Sea ice extent also has been linked to larval krill overwintering survival and
recruitment (Daly and Macaulay, 1988; Marschall, 1988; Daly, 1990; Kawaguchi and Satake,
1994). Elevated numbers of first year juvenile krill have been observed following years with
heavy winter ice cover, while juvenile abundances are generally lower following years with
low sea ice extent (Siegel and Loeb, 1995). Winter sea ice has been proposed to provide
larvae with a refuge from predators and sea ice biota as a primary food source (Daly, 1990).
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However, during both winters of our study, sea ice biota concentrations were very low (0.05 0.07 mg chl m-3) at the ice-water interface where larval krill feed (Daly, 2004). In addition, a
large percentage of larvae were not associated with sea ice, particularly in 2002. Thus, the
presence of sea ice is not necessarily a good predictor of food availability for overwintering
larvae. At these high latitudes, summer food availability contributing to the robust condition
of larvae prior to overwintering may be more important. Nevertheless, sea ice biota can be an
important source of food during spring in Marguerite Bay (Daly, 2004).
Finally, ice edge blooms are suggested to be important for krill reproduction in the
vicinity of the Antarctic Peninsula (Siegel and Loeb, 1995; Ross and Quetin 1986). Ice edge
retreat is often tightly coupled to spring phytoplankton blooms in Antarctic waters (Sullivan
et al., 1988; Arrigo and McClain, 1994; Garibotti et al., 2005), whereby phytoplankton
blooms may develop about 2 weeks after sea ice recedes from a particular location. Ice melt
creates a fresh surface layer which increases water column stability, resulting in a shallower
mixed layer in which cells experience higher irradiance. These processes coupled to the
availability of nutrients in surface waters fuel phytoplankton production. Our results,
however, suggest that the formation of spring phytoplankton blooms west of the Antarctic
Peninsula is not necessarily coupled to the retreat of the ice edge, particularly in the northern
half of the Peninsula. Instead, phytoplankton blooms first appear near the shelf-break and
gradually progress to more coastal areas, suggesting that shelf-break processes could be
controlling phytoplankton growth in this region. Antarctic surface waters are rich in
macronutrients; however, iron deficiency has been proposed as a factor possibly limiting
phytoplankton growth (De Baar et al., 1995; Holm-Hansen et al., 2004; 2005). The ACC
flows eastward at relatively high velocities, particularly at frontal areas such as the Polar
Front and the Southern ACC Front. The strong current interacts with the bathymetry when it
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encounters the shelf-break generating meanders that can usually be detected at the surface.
This interaction of the ACC with bottom features has the potential of bringing iron-rich deep
waters to the surface and supply the necessary nutrients to sustain the development of
phytoplankton blooms. The upwelling of iron-rich deep ACC waters has been described for
other regions of the Southern Ocean, including the Scotia Sea, the Polar Frontal region
downstream of South Georgia and the Ross Sea (De Baar et al., 1995; Measures and Vink,
2001; Holm-Hansen et al., 2005). We hypothesize that the upwelling of iron-rich deep water
at the shelf-break (De Baar et al., 1995; Holm-Hansen et al., 2004; 2005), rather than the
retreat of sea ice may be a major factor controlling the formation of spring phytoplankton
blooms in offshore waters of the Drake Passage and along the shelf break of the Antarctic
Peninsula. In the southern regions of the Antarctic Peninsula, the receding ice edge and
chlorophyll accumulations coincide spatially with about a 2 - 4 week lag and it is harder to
determine whether sea ice melting, upwelling of iron-rich waters, or a combination of both,
are controlling phytoplankton growth. However, previous evidence suggests that spring
blooms are not related to the ice edge in the Bellingshausen Sea, possibly because the ice
retreats rapidly in this area, not allowing enough time for a surface meltwater layer to build up
and thus preventing the development of a stability-induced phytoplankton bloom (Savidge et
al., 1995).
Summary
The analysis of typical chlorophyll distributions west of the Antarctic Peninsula from 1997
through 2004 revealed that the southernmost sectors of the region, such as the eastern
Bellingshausen Sea and Marguerite Bay, are considerably more productive in terms of
phytoplankton growth than any other area analyzed, including the continental shelf along the
17
Peninsula and the western Scotia Sea. These southern sectors support elevated chlorophyll
concentrations from October until March, providing the food levels required by Antarctic krill
for a successful reproduction and larval survival. In particular, the variability observed in
phytoplankton distributions during the spring - summer 2000/2001 and 2001/2002 can be
related to the strong differences observed in krill abundances between fall 2001 and 2002,
respectively. The 2000/2001 season was more productive than 2001/2002. High chlorophyll
concentrations in the vicinity of Marguerite Bay and the Bellingshausen Sea were observed
from January through March, and coincided with very high krill abundances recorded in
oceanic and coastal waters the following fall. In contrast, 2001/2002 showed reduced
chlorophyll concentrations during summer and lower overall krill numbers during fall
throughout the study area. Differences in sea ice conditions in Marguerite Bay likely
contributed to the variable krill numbers observed in this area. Although ice conditions were
similar during both winters, sea ice persisted throughout the spring - summer 2001/2002,
preventing the formation of diatom blooms which are the main food source for krill in coastal
waters during the productive months. In contrast, sea ice melted early in 2000/2001, allowing
elevated chlorophyll concentrations to develop within the Bay by December - January. Both
the presence of above average chlorophyll concentrations in the Bellingshausen Sea and
outside of Marguerite Bay during krill’s reproductive season (November - March) and
favorable sea ice conditions in coastal areas during summer - fall likely facilitated the
increased krill abundances observed during 2001. The formation of phytoplankton blooms in
offshelf waters during spring was not related to the retreat of the ice edge in our study area,
except possibly in the southern regions. It is likely that other processes taking place at the
shelf break, such as the upwelling of iron-rich deep waters, are influencing the development
of these blooms.
18
Acknowledgements
This study was supported by the US National Science Foundation (NSF) grants OPP-9910610
and OPP-196489 to K. Daly, and by US NASA grant NNS04AB59G to C. Hu. We are
grateful to K. Arrigo for providing the daily sea ice data for Marguerite Bay and to E.
Chapman for making the monthly ice edge data available. We also thank Brock Murch of
USF/IMaRS for his assistance in obtaining and processing the SeaWiFS satellite data. This
publication represents GLOBEC contribution #xxx.
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Figure 1. Length (mm) and frequency (ind m-3) distribution of krill west of the Antarctic
Peninsula during (a) fall 2001 and (b) fall 2002.
Figure 2. Location of the study area and geographic references. The dashed line represents
the 1000 m isobath.
Figure 3. Bi-weekly climatology (1997 - 2004) of SeaWiFS chlorophyll-a concentrations (mg
m-3) between September and March. White areas indicate no data.
Figure 4. (a) Locations of the 14 regions along the western Antarctic Peninsula overlaid over
the 7-year climatology (1998 - 2004) of SeaWiFS chlorophyll concentrations during January
1 - 14. (b) The median chlorophyll concentration at each region for each 2-week period
during the 2000/2001 (black dots) and 2001/2002 (grey squares) spring – summer seasons.
Concentrations lower than 0.01 mg m-3 and greater than 20 mg m-3 were excluded from the
calculations. The 7-year climatology (median) is also shown for each region (black solid
line).
Figure 5. Biweekly anomalies of chlorophyll concentrations (mg chl above or below the 7year climatology) for late November, December, January and February of 2001 (first column
of images) and 2002 (second column). Black regions indicate no data due to the presence of
clouds and/or sea ice.
Figure 6. Bi-weekly SeaWiFS chlorophyll-a concentrations in October (Oct), November
(Nov) and December (Dec) of (a) 2000 and (b) 2001. The mean monthly location of the ice
24
edge is also shown: red line is the location of the ice edge during the previous month, and
yellow line is for the current month. White line: 1000 m isobath.
Figure 7. (a) Two regions in the Marguerite Bay were chosen to analyze the ice data, where Daily
ice-free area (km2) for the northern and southern bay segments during 2001 (red solid line) and
2002 (blue broken/dotted line) are shown in (b) and (c), respectively. The 7-year daily
climatology (1997 - 2004) is also shown (black broken line).
25
Tables
Table 1. The 14 selected regions in the study area. ACC: Antarctic Circumpolar Current.
Region Location
System type
Depth
1
ACC - Bellingshausen Sea
Oceanic
> 2000 m
2
ACC – Drake Passage
Oceanic
> 2000 m
3
ACC - Scotia Sea
Oceanic
> 2000 m
4
Bellingshausen Sea - Offshelf
Oceanic
> 2000 m
5
Antarctic Peninsula - Offshelf
Oceanic
> 2000 m
6
Scotia Sea - Offshelf
Oceanic
> 2000 m
7
Bellingshausen Sea
Shelf break
500-2000 m
8
Antarctic Peninsula
Shelf break
500-2000 m
9
Bellingshausen Sea - Southern Ant. Pen. Continental shelf
< 500 m
10
Mid-Antarctic Peninsula
Continental shelf
< 500 m
11
Northern Antarctic Peninsula
Continental shelf
< 500 m
12
Scotia Sea – Weddell Sea
Cont. shelf & shelf break ~ < 2000 m
13
Northern Marguerite Bay
Continental shelf
< 500 m
14
Southern Marguerite Bay
Continental shelf
< 200 m
26
Figures
Figure 1
27
Figure 2
28
29
30
Figure 3
(a)
Figure 4a
31
Figure 4b
32
Figure 5
33
(a)
34
(b)
Figure 6
35
Figure 7
36